Geometry-Modulated Magnetoplasmonic Optical Activity of Au

Subscriber access provided by PEPPERDINE UNIV

Communication

Geometry Modulated Magnetoplasmonic Optical Activity of Au Nanorod Based Nanostructures Bing Han, Xiaoqing Gao, Lin Shi, Yonglong Zheng, Ke Hou, Jiawei Lv, Jun Guo, Wei Zhang, and Zhiyong Tang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b02583 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Geometry Modulated Magnetoplasmonic Optical Activity of Au Nanorod Based Nanostructures Bing Han,†,‡ Xiaoqing Gao,‡ Lin Shi,‡ Yonglong Zheng,‡ Ke Hou,‡ Jiawei Lv,‡ Jun Guo,‡ Wei Zhang*,§ and Zhiyong Tang*,‡ †

College of Environmental Science and Engineering, North China Electric Power University,

Beijing 102206, P. R. China ‡

CAS key laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in

Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China §

Institute of Applied Physics and Computational Mathematics, Beijing 100088, P. R. China

ABSTRACT: Comprehension and modulation of optical activity at nanoscale have attracted tremendous interest in the past decades due to its potential application in many fields including chemical/biological sensing, artificial metamaterials, asymmetric catalysis, etc. As for the conventional molecular materials, magnetic field is among the most effective routes in inducing and manipulating their optical activity; whereas the magnetic optical activity at nanoscale calls for deeper understanding, especially for anisotropic noble metal nanoparticles. In this work, distinctly different magnetic circular dichroism (MCD) responses are demonstrated in gold nanorods (GNRs) with a derivative-shaped MCD signal corresponding to the transverse surface plasmon resonance (TSPR) band and a Gaussian-shaped signal at the position of the longitudinal surface plasmon

1 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 23

resonance (LSPR) band. Furthermore, changing the aspect ratio of GNRs easily regulates such magnetoplasmonic CD response. More interestingly, GNR assemblies with different geometric configuration (end-to-end and side-by-side) show structure-sensitive magnetoplasmonic CD response. Armed with theoretical calculation, we clearly elucidate the intrinsic relationship of the resultant magnetoplasmonic CD response with the optical symmetry and geometry factor inside one-dimensional GNRs. This work not only greatly benefits our understanding towards the nature of SPR mode in anisotropic plasmonic nanostructures but also opens the way to achieve tunable magnetoplasmonic response, which will significantly advance the design and application of optical nanodevices.

KEYWORDS: Au nanorods; self-assembly; geometry; magnetic circular dichroism; optical activity

Plasmonic circular dichroism (CD) response, the emerging optical activity at nanoscale, has become one of the hottest research topics in chemistry, physics and materials due to its scientific and technological importance.1-4 Such optical activity is generated either by conjugation of noble metal nanoparticles (NPs) with chiral molecules or by constructing noble metal NP assemblies with chiral arrangement.5-10 Therefore, analogously to conventional molecular CD spectrum, plasmonic CD in noble metal nanostructures can be also classified into natural CD that requires chiral or helical distribution of electric charges in space.11,12 In contrast, there is an external and general way to generate CD activity in matters, which has been largely ignored in nanostructures. By applying a magnetic field parallel to the direction of light propagation, all matters regardless of with or without chirality exhibit CD effect under circularly polarized light.13,14 The universal optical activity is termed as magnetic circular dichroism (MCD), which originates from breaking


Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

of time-reversal symmetry by a magnetic field. On this basis, external magnetic field would be expected to provide a general and powerful route to achieve plasmonic optical activity at nanoscale.15,16 Noteworthily, at molecular scale, MCD spectroscopy has shown the unique capability to reveal the degeneracy information of electronic energy levels, which cannot be derived from CD spectroscopy or light absorption spectroscopy.17-20 It is known that the degeneracy of excited states is directly affected by the symmetry of molecules, so MCD signals are highly sensitive to the structural change of molecules.13,19 By far, comprehension of molecular MCD is quite mature. However, knowledge of the origin of MCD response at nanoscale stays in the preliminary stage. Especially, the existent experimental design and theoretical study have mostly focused on the simple model with spherical NPs.21-23 Therefore, the first imperative question to be answered is whether MCD spectroscopy is effective in revealing symmetry in nanosystem. It has been widely studied that symmetry modulation in noble metal NPs gives rise to intricate plasmonic resonance behavior.24-26 Particularly, anisotropic noble metal NPs, which possess multiple surface plasmon resonance (SPR) bands corresponding to electron oscillation along different geometrical directions,27-29 have exhibited fantastic magnetoplasmonic properties.30,31 Followed by the first report on the experimental MCD spectra of gold nanorods (GNRs),32 several pioneering works have verified that noble metal nanorods are excellent candidates to study the relationship between geometry/symmetry of NPs and the resultant magnetoplasmonic CD response.30,33 Furthermore, recent studies have convincingly indicated the importance of both the geometric anisotropy of ferromagnetic elliptical nanodisk and its periodic arrangement induced electromagnetic coupling in actively manipulating the magneto optical activity (magneto-optical Kerr effect).34-36 Enlightened by these studies, another critical question is whether the


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 23

magnetoplasmonic CD response in noble metal nanostructures can be modulated by manipulating geometry factor such as shape, dimension and inter-particle arrangement of constituent NPs. Unlike spherical NPs that are highly symmetric in both structure and interaction,21,23 anisotropic noble metal NPs like GNRs have the unique ability to assemble into a number of different aligned configurations, leading to intricate but interesting optical anisotropy.37-41 This configuration modulated optical property of GNR assemblies across the visible and near-infrared (NIR) spectral region makes them the ideal model to explore the key factors in manipulation of magnetoplasmonic CD, which will significantly benefit future development in active plasmonics. Hence, GNRs are selected in this work to thoroughly study the magnetoplasmonic CD effect at nanoscale. Impressively, distinct MCD response is discerned in GNRs for SPR modes along different directions. GNRs display a derivative-shaped MCD signal corresponding to the transverse SPR (TSPR) band, while a Gaussian-shaped signal appears at the position of the longitudinal SPR (LSPR) band. Both experimental result and theoretical simulation reveal that the geometry/symmetry dependent SPR coupling of TSPR and LSPR bands gives rise to the different magnetoplasmonic effect. Moreover, such magnetoplasmonic CD signal can be manipulated either by tuning the aspect ratio (AR) of GNRs or by fabricating GNR assemblies with different configurations, i.e. side-by-side (SS) and end-to-end (EE). The crucial factor in both modulation routes is explored to be the geometry factor in GNRs or GNR assemblies.


Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 1. (a) Transmission electron microscopy (TEM) image and (b) MCD spectrum (top curve) and UV-Vis-NIR spectrum (bottom curve) of well-dispersed GNSs; (c) TEM image and (d) MCD spectrum (top curve) and UV-Vis-NIR adsorption spectrum (bottom curve) of well-dispersed GNRs with AR of 3.0, scale bar in (a) and (c) is 100 nm. Note that all the MCD spectra are plotted in term of ΔA (difference between absorbance of left and right circularly polarized light in the presence of a magnetic field) divided by ASPR/LSPR (absorbance of SPR peak for GNSs or LSPR peak for GNRs) and B (magnitude of the magnetic field), and MCD spectra are measured under the applied magnetic field of B = 1.6 T. All the UV-Vis-NIR adsorption spectra are plotted in term


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 23

of A (absorbance) divided by ASPR/LSPR (absorbance of SPR peak for GNSs or LSPR peak for GNRs). GNRs were synthesized following the previously-reported seed-mediated growth method42, while spherical gold nanoparticles (GNSs) were also prepared as the contrast samples. As shown in Figure 1a and 1c, both as-synthesized GNSs and GNRs are well dispersed in aqueous solution and have narrow size distribution. The GNSs of 18.7 nm in diameter exhibit a sharp SPR adsorption band at around 520 nm (bottom curve in Figure 1b). Correspondingly, their magnetoplasmonic CD signal is a derivative shaped peak with its crossing zero point at the position of the SPR band (~520 nm) (top curve in Figure 1b), originating from interaction of the circular plasmonic modes with an external magnetic field.21 In detailed analysis, the original degenerate circular plasmonic modes in GNSs split under the magnetic field along light propagation direction; as a result, GNSs adsorb left and right circularly polarized light (LCP and RCP) in different frequency, resulting in the derivative lineshape. As comparison, GNRs with an AR of 3.0 (length and diameter are 46.7 nm 15.6 nm, respectively) show obviously different MCD response for their TSPR and LSPR bands. Top curve in Figure 1d manifests that GNRs also possess a derivativeshaped signal near the TSPR band, which is quite similar to the magnetoplasmonic CD signal of GNSs. However, at the position of LSPR band, the magnetoplasmonic CD response of GNRs is of Gaussian lineshape. As further indicated in Figure S1, when the magnetic field is switched oppositely, MCD signal of GNRs is completely reversed while their UV-Vis-NIR adsorption peak remains unchanged, confirming that the MCD signals result from real magneto-optical response rather than experimental artifact. Also, the intensity of MCD signal is linearly dependent on the magnitude of applied magnetic field, implying highly magnetic field sensitive magnetoplasmonic CD response in GNRs (Figure S2). It is noteworthy that such distinctive spectral behavior for


Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

TSPR and LSPR modes in GNRs have never been recognized in traditional spectroscopy such as UV-Vis-NIR spectra, in which only two single peaks are discerned at different wavelengths due to the plasmon oscillation along either longitudinal or transverse axis. Therefore, the rich magnetoplasmonic CD response in GNRs system must reflect additional information related to the nature of SPR mode, which is stimulated by the external magnetic field.

Figure 2. TEM images of GNRs with aspect ratio of (a) 2.1, (b) 2.4, (c) 3.0, (d) 3.7, (e) 3.9; scale bar = 100 nm, and corresponding MCD spectra (f) and UV-Vis-NIR spectra (g) of well-dispersed GNRs with different aspect ratio. MCD data are measured under the applied magnetic field of B = 1.6 T. Such interesting finding in MCD signal pushes us to explore the intrinsic relationship between the magnetoplasmonic property and the geometry factor of GNRs. Then, GNRs of different AR from 2.1 to 3.9 are synthesized, and Figure S3 and Table S1 summarize the statistical size distribution of as prepared GNRs. It is clear that all the GNRs with varied AR are very uniform


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 23

with narrow size distribution. The subsequent examination by MCD spectroscopy draws two important conclusions (Figure 2): (1) With the AR of GNRs increasing, the bisignate magnetoplasmonic CD peaks corresponding to the TSPR band keep almost unchanged, whereas the Gaussian-shaped MCD signals red shift with the LSPR bands (curve in Figure 2f). It is noticed that the shift of the magnetoplasmonic CD response shares the same AR-dependent change with the UV-Vis-NIR adsorption peaks (curves in Figure 2g), which convincingly demonstrate that the magnetoplasmonic CD signal originates from interaction between the SPR modes and an external magnetic field. (2) When the AR of GNRs increases, the anisotropic factor (g factor) for either the transverse or longitudinal magnetoplasmonic CD signals decreases (Figure 2f and Table S3). The decreasing trend of the MCD intensity with AR increasing is quite different from that of the light absorption of GNRs, which is of enhanced extinction coefficient ε with AR increasing.43

Figure 3. Scheme of symmetry of (a) TSPR or LSPR mode in GNRs, and (b) proposed MCD response corresponding to TSPR (blue filled area) and LSPR band (orange filled area); the red curve in (b) represents the experimental MCD signal. Why does distinct magnetoplasmonic CD responses appear for different SPR modes of GNRs, and what is the reason causing decreased MCD intensity with AR increasing? Similar MCD


Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

lineshape difference is discerned in triangular silver nanoprisms, which is attributed to the dipolar and quadrupolar SPR modes, respectviely.31 In contrast, both TSPR and LSPR modes of GNRs originate from dipolar resonance,44-46 which is totally different from those in Ag nanoprisms. Thus, there must exist an unexplored and major factor contributing to the distinct magnetoplasmonic CD signals at the position of TSPR and LSPR band for GNRs. Since MCD spectroscopy of molecules has been widely studied, the well accepted principles may be adopted to give a preliminary explanation on the magnetoplasmonic CD response of GNRs. As for molecules, lineshape of their MCD response is known to be determined by degeneracy of the excited or ground state. On one hand, if a molecule possesses a proper or improper rotation axis of the order greater than three, excited-state degeneracy is possible.13 The degenerate excited states will give rise to derivativeshaped MCD signal, which is recognized as “A term”.13,47 On the other hand, under magnetic field, the excited states might couple with each other, which results in Gaussian-shaped “B term”. In the case of low-symmetry molecules with rotation axis of the order two or one, there will be only nondegenerate excited states, so only “B terms” are expected in the MCD spectra.13,47 The intensity of “B term” is inversely proportional to the energy difference between the coupled states. Accordingly, TSPR and LSPR modes of GNRs can be identified as the excited states in NPs, because they originate from oscillation of free electrons along the transverse or longitudinal axis. The different modes have different symmetry because of the anisotropic shape of GNRs. Seen from the left scheme in Figure 3a, GNRs possess cylindrical symmetry for rotation along the longitudinal axis. As comparison, the rotational symmetry is lowered for rotation along the transverse axis, which is a 2-fold rotational axis (right scheme in Figure 3a). Therefore, the LSPR mode of a GNR is nondegenerate whereas its TSPR mode is doubly degenerate.48 LCP and RCP excite circular TSPR modes of GNRs, which are doubly degenerate in the absence of magnetic field. When an external


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 23

magnetic field is applied along the direction of circularly polarized light propagation, the degeneration is lifted. As shown in Figure 3b, the adsorption of LCP and RCP for the two circular TSPR modes is shifted in energy, leading to a derivative-shaped signal near the position of the TSPR adsorption peak (blue filled areas in Figure 3b). It is noted that the origin of the magnetoplasmonic CD signal for the TSPR mode is quite similar with that of the GNSs, which exhibit rotational symmetry at any directions. But for the non-degenerate LSPR mode, the derivative-shaped signal will not be observed. Under magnetic field, the non-degenerate LSPR mode could couple with TSPR mode, generating a negative peak at the position of the LSPR mode and a positive peak at the position of TSPR mode (orange filled areas in Figure 3d). The rationalization of regarding the interaction between TSPR mode and LSPR mode under magnetic field as B-like term has been verified in the recent work.30 Moreover, when the AR of GNRs increases, the energy difference between LSPR and TSPR becomes larger. Thus, the MCD signal arising from the magnetic field induced coupling will decrease in magnitude due to the weaker coupling between LSPR and TSPR.


Page 11 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 4. TEM images of SS GNR assembly (a) and EE GNR assembly (c); scale bar = 50 nm. CD spectra (top curves) and UV-Vis-NIR spectra (bottom curves) of SS (red curves in (b)) or EE (pink curves in (d)) assembled GNRs with AR of 2.3. Blue curves stand for the corresponding spectra of dispersed GNRs, as reference. MCD data are measured under the applied magnetic field of B = 1.6 T. The fact that GNRs well dispersed in solution exhibit geometry dependent magnetoplasmonic CD response encourages us to further investigate how the conformation of GNR assemblies affects the resultant MCD signal. With its anisotropic shape, dispersed GNRs are able to assemble into different patterns in solution with help of organic linkers, among which end-to-end (EE) and side-


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 23

by-side (SS) are mostly studied. GNR assemblies have shown the conformation dependent optical property.30,49 Here we monitor the MCD response in EE and SS assembly. UV-Vis-NIR absorption spectra (bottom curves in Figure 4b and 4d) and transmission electron microscopy (TEM) images (Figure 4a and 4c) confirm the successful formation of EE and SS GNR assemblies. For instance, as indicated by the bottom red curve in Figure 4b, compared with the UV-Vis-NIR adsorption spectrum of dispersed GNRs (bottom blue curve in Figure 4b), the intensity of the LSPR adsorption decreases and blue shifts from 654 nm to 622 nm, which is characteristic of SS assembled GNRs.49 Correspondingly, the longitudinal MCD peak of SS GNR assembly blue shifts and its g factor increases compared with the dispersed GNRs, while the transverse MCD signal red shifts (top red curve in Figure 4b). In regard of EE GNR assembly, its LSPR peak displays the considerably reduced intensity and bathochromic shift from 672 nm to 762 nm (bottom pink curve in Figure 4d).49 Accordingly, the longitudinal MCD peak of EE GNR assembly shows the obviously reduced intensity and red shift to 770 nm (top pink curve in Figure 4d). To obtain full comprehension, GNRs with different AR are used as building blocks for assembly (Figure S4 and Table S2). As displayed in Figure S5-S8, for all the GNRs used, similar change tendency in MCD signals is distinguished. Very interestingly, the quantitative analysis on MCD spectra of GNR assemblies reveals that g factor of the GNR assembly has different change compared with the dispersed GNR. Because the strong MCD modulation appears for the LSPR mode, the longitudinal magnetoplasmonic CD response is selected for comparison. As summarized in Table S4 and S5, g factor of the longitudinal magnetoplasmonic CD signal is enhanced after GNRs form SS assembly, whereas it is reduced in EE GNR assembly. It can be concluded that for GNR assemblies, when the LSPR moves to lower energy, the magnetoplasmonic CD signal decreases; conversely, the


Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

MCD intensity increases. Evidently, the similar trend is found for dispersed GNRs, in which magnetoplasmonic CD response decreases along with red shifted LSPR due to the increased AR. Altogether, one can clearly recognize the similarity between molecular MCD and magnetoplasmonic CD response. On one hand, taking the basic principle in MCD theory, the analysis on the MCD response of dispersed GNRs is reasonable. On the other hand, both dispersed GNRs and GNR assemblies have shown geometry or conformation dependent magnetoplasmonic response. However, it must be pointed out that though molecular MCD and magnetoplasmonic CD are all based on symmetry principle, there are several different aspects between them. The traditional MCD theory deals with transition in molecules, but surface plasmon in noble metal NPs cannot be considered as transition. What’s more, in GNR assemblies, the existence of coupling between NRs makes the explanation on their magnetoptical activity more complicated, which is very difficult to be deduced from basic MCD theory. Therefore, we develop a theory to understand the magnetoplasmonic CD response of dispersed GNRs as well as how the conformation of GNR assemblies influence the resultant MCD effect. We first consider the case of the wavevector of circularly polarized light along the z direction and the long axis of a GNR along x direction (Figure S9). The MCD signal of GNRs comes from the magnetic field induced difference of optical adsorption between LCP and RCP, which can be expressed as:

MCDGNR Abs LCP Where Abs

Abs RCP (1)

εm ωV Im E αE , ω is the frequency of the incident light,

with = 1

for LCP or RCP, ̂ ̂

(2)


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0

In Equation 2, is the unit matrix,

0

,

Page 14 of 23

and

are the geometry factor for SPR

mode along the longitudinal and transverse direction, respectively. is the volume of the GNR, are the dielectric constants of Au and the medium, respectively. By combining Equation 1

and

and Equation 2 (more details in part S3.1 in Supporting Information), we obtain the following equations: η=1

Abs (η =

1) =

4

Im

Δ

(3)

Where Δ =(

+

1

)⋅(

here

+

1

(

)

4

is the cyclotron frequency). As for GNRs with large AR, the

anisotropic factor at LSPR is: ∝

(5)

Notably, based on our MCD theory, one can see that the MCD signal is proportional to the magnetic field (in Equation 3 and 4, (

and

is proportional to B), and strongly depends on the geometry

) of the structure. MCD peaks are determined by Re∆

0. It implies that MCD signal

is attributed to the interplay of the SPR modes along different directions. For a sphere like GNS (

), the original degenerate SPR modes split under magnetic field and adsorb

resonantly LCP or RCP at different frequency (

or

, part S3.1 in Supporting Information)

near the SPR mode. But for elongated particles like GNRs, the threefold degeneracy of these modes is lifted with doubly degenerate transverse SPR mode and non-degenerate longitudinal SPR mode. For this case,

, MCD signals appear at both LSPR mode and TSPR mode of


Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

GNRs. As a result, for GNSs, a derivative-shaped signal can be observed with its crossing zero corresponding to the SPR adsorption peak. However, with respect to GNRs, the shape anisotropy gives rise to TSPR and LSPR modes at different resonant frequency. On one hand, corresponding to the TSPR adsorption peak, a derivative-shaped MCD signal is produced due to the double degeneracy. On the other hand, coupling between LSPR and TSPR mode is induced by external magnetic field, which leads to new MCD signals located at the frequency of LSPR and TSPR adsorption peak (part S3.1 in Supporting Information). To sum up, the MCD response of GNRs is a bisignate peak at the position of TSPR adsorption peak and a single peak located at the frequency of LSPR adsorption (Figure 1d and 3b). To understand the dependence of the optical property of GNRs on their geometry like AR is quite important. Figure 5 presents theoretical simulation of MCD signals of GNRs with varied AR, which reproduces the lineshape of the experimental data very well (Figure 2f). As the AR of GNRs becomes larger, the geometry factor 1

1/

1

decreases (

ln

, and

). As demonstrated by the explicit dependence of the g factor on the geometry factor

of GNRs in Equation 5, the reduced geometry factor

correlates with the decreased g factor. This

is also consistent with the fact that as for GNRs with larger AR, the increased difference between and

leads to the decreased longitudinal MCD (

∝

/

/

) (Figure S9b), which

is caused by the weaker coupling between the LSPR and TSPR modes under magnetic field. Impressively, the quantitatively good agreement of g factor (up to an overall constant) between experiment and theory confirms the credibility of our model (Table S3).


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 23

Figure 5. Calculated MCD spectra of dispersed GNRs with different AR. In regard of GNR assemblies, the existence of coupling between GNRs must be considered (part S3.2 in Supporting Information). A pair of coupled GNRs is considered with dipole moment

:

1,2 (6) where

is the incident field and

is the polarizability. Taking into account the electromagnetic

coupling between the dipoles, the dipole moment can be simplified as ,

⋅

, with

, L is the geometry factor of dispersed GNRs, and q is a constant

determined by the assembly configuration. Therefore, the coupling between GNRs produces an effective geometry factor Leff . For the SS configuration, q

1 and thus Leff

L, resulting in the

blue shift of longitudinal MCD peak with increased g factor; while for EE configuration, q = and hence Leff

2

L, leading to the red shift of longitudinal MCD peak with decreased g factor. This

analysis is well consistent with experimental observations (Figure 4b and 4d). In the more accurate theoretical model, the effect of multiple poles is taken into account by renormalization of the parameter q. By using fitting parameters for SS or EE configuration, excellent agreement between theory and experiment has been obtained, as demonstrated in the Table S4 and S5. In overall, the


Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

conformation dependent MCD response in GNR assembly is ascribed to increase or decrease of the effective geometry factor in SS or EE configuration, respectively. This is coincident with the influence of geometry factor on the resultant MCD response in dispersed GNRs. In conclusion, geometry modulated magnetoplasmonic CD response is explored in welldispersed GNRs and GNR nano-assemblies. The anisotropic shape of GNRs leads to different symmetry between TSPR and LSPR modes, which gives rise to MCD response with distinct lineshape. The direct relationship between symmetry of SPR mode and the spectral information is well established through MCD spectroscopy, which has never been recognized in previous UVVis-NIR or CD spectra. The position and magnitude of such magnetoplasmonic CD response can be further manipulated by either tuning the AR of GNRs or fabricating GNR assemblies with different configurations. Both experimental and theoretical results disclose that the geometry factor in GNRs or nano-assemblies is the crucial factor to produce highly tunable magnetoplasmonic CD signals. Our work highlights that MCD spectroscopy will be a powerful method to discriminate SPR modes with different symmetry in anisotropic metal nanostructures. Furthermore, understanding the importance of geometry factors lays the foundation to achieve sophisticated modulation of the magnetoplasmonic response at nanoscale, which will greatly benefit their applications in nanophotonic devices and refractometric sensing.

ASSOCIATED CONTENT Supporting Information. Materials and Methods in Part S1: Synthesis of GNRs and controlled assembly of GNRs into SS mode and EE mode. Experimental data of GNRs or GNR assemblies with different AR in Part S2: Figure S1: MCD spectra and UV-Vis-NIR spectra of well-dispersed


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

GNRs with different AR under magnetic field at 1.6 T or -1.6 T. Figure S2: MCD spectra of welldispersed GNRs with AR of 3.0 under different magnitude of magnetic field. Figure S3: Typical TEM images and corresponding AR distribution of five GNRs. Table S1: Statistics of the dimension of GNRs with different AR. Figure S4: Typical TEM images and corresponding AR distribution of five GNRs used in assembly. Table S2: Statistics of the dimension of GNRs with different AR used for assembly. Figure S5~S8: TEM images, MCD spectra and UV-Vis-NIR spectra of SS GNR assembly and EE GNR assembly using GNR with different AR. Theoretical calculation details of MCD theory for well-dispersed Au nanorods (GNRs) and GNR assemblies in Part S3: Figure S9: Diagram of GNR-CPL interaction model. Table S3: Comparison between the experimental and theoretical results of g factor for dispersed GNRs with different AR. Table S4 and S5: Comparison between the experimental and theoretical results of g factors for SS GNR assembly and EE GNR assembly (PDF). AUTHOR INFORMATION Corresponding Author [email protected] [email protected] Author Contributions All authors have given approval to the final version of the manuscript. Funding Sources ACKNOWLEDGMENT


Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

This work was supported by National Key Basic Research Program of China (Grant Nos. 2014CB931801 and 2016YFA0200700, Z.Y.T.; 2013CB632805 and 2017YFA0303400, W.Z.), National Natural Science Foundation of China (Grant Nos. 21475029, 91427302, Z.Y.T.; 11174042, 11374039 and 11774036, W.Z.; 21703064, B.H.), Science Fund for Creative Research Groups of the National Natural Science Foundation of China (21721002, Z.Y.T.), Frontier Science Key Project of the Chinese Academy of Sciences (Grant No. QYZDJ-SSW-SLH038, Z.Y.T.), Instrument Developing Project of the Chinese Academy of Sciences (Grant No. YZ201311, Z.Y.T.), CAS-CSIRO Cooperative Research Program (Grant No. GJHZ1503, Z.Y.T.), the “Strategic Priority Research Program” of Chinese Academy of Sciences (Grant No. XDA09040100, Z.Y.T.), K.C.Wong Education Foundation (Z.Y.T.) and the Fundamental Research Funds for the Central Universities (Grant No. 2017MS046, B.H.). Notes The authors declare no competing financial interests. REFERENCES (1) Gansel, J. K.; Thiel, M.; Rill, M. S.; Decker, M.; Bade, K.; Saile, V.; Von Freymann, G.; Linden, S.; Wegener, M. Science 2009, 325, 1513. (2) Kuzyk, A.; Schreiber, R.; Fan, Z. Y.; Pardatscher, G.; Roller, E. M.; Hogele, A.; Simmel, F. C.; Govorov, A. O.; Liedl, T. Nature 2012, 483, 311. (3) Guerrero-Martínez, A.; Alonso-Gómez, J. L.; Auguié, B.; Cid, M. M.; Liz-Marzán, L. M. Nano Today 2011, 6, 381. (4) Ben-Moshe, A.; Maoz, B.; Govorov, A. O.; Markovich, G. Chem. Soc. Rev. 2013, 42, 7028. (5) Alivisatos, A. P.; Mastroianni, A. J.; Claridge, S. A. J. Am. Chem. Soc. 2009, 131, 8455.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 23

(6) Bao, Z. Y.; Zhang, W.; Zhang, Y. L.; He, J.; Dai, J.; Yeung, C. T.; Law, G. L.; Lei, D. Y. Angew. Chem. Int. Ed. 2017, 129, 1303. (7) Kuzyk, A.; Schreiber, R.; Zhang, H.; Govorov, A. O.; Liedl, T.; Liu, N. Nat. Mater.2014, 13, 862. (8) Lan, X.; Lu, X.; Shen, C.; Ke, Y.; Ni, W.; Wang, Q. J. Am. Chem. Soc. 2015, 137, 457. (9) Li, Z. T.; Zhu, Z. N.; Liu, W. J.; Zhou, Y. L.; Han, B.; Gao, Y.; Tang, Z. Y. J. Am. Chem. Soc. 2012, 134, 3322. (10) Ma, W.; Kuang, H.; Xu, L.; Ding, L.; Xu, C.; Wang, L.; Kotov, N. A. Nat. Commun. 2013, 4, 2689. (11) Fan, Z.; Govorov, A. O. Nano Lett. 2010, 10, 2580. (12) Govorov, A. O.; Fan, Z. Y.; Hernandez, P.; Slocik, J. M.; Naik, R. R. Nano Lett. 2010, 10, 1374. (13) Mason, W. R. A Practical Guide to Magnetic Circular Dichroism Spectroscopy; WileyInterscience: New York, 2007. (14) Stephens, P. J. J. Chem. Phys. 1970, 52, 3489. (15) Armelles, G.; Caballero, B.; Prieto, P.; Garcia, F.; Cebollada, A.; Gonzalez, M. U.; GarciaMartin, A. Nanoscale 2014, 6, 3737. (16) Armelles, G.; Cebollada, A.; García-Martín, A.; González, M. U. Adv. Opt. Mater. 2013, 1, 10. (17) Bradley, J. M. J. Am. Chem. Soc. 2011, 133, 19676. (18) Kobayashi, N.; Nakai, K. Chem. Commun. 2007, 40, 4077. (19) Muranaka, A.; Yokoyama, M.; Matsumoto, Y.; Uchiyama, M.; Tsuda, A.; Osuka, A.; Kobayashi, N. Chem. Phys. Chem. 2005, 6, 171.


Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(20) Ziegler, C. J.; Erickson, N. R.; Dahlby, M. R.; Nemykin, V. N. J. Phys. Chem. A 2013, 117, 11499. (21) Pineider, F.; Campo, G.; Bonanni, V.; Fernandez, C. J.; Mattei, G.; Caneschi, A.; Gatteschi, D.; Sangregorio, C. Nano Lett. 2013, 13, 4785. (22) Shiratsu, T.; Yao, H. J. Nanophotonics 2016, 10, 046004. (23) Yao, H.; Shiratsu, T. Nanoscale 2016, 8, 11264. (24) Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. Science 2003, 302, 419. (25) Lassiter, J. B.; Sobhani, H.; Fan, J. A.; Kundu, J.; Capasso, F.; Nordlander, P.; Halas, N. J. Nano Lett. 2010, 10, 3184. (26) Wang, H.; Brandl, D. W.; Nordlader, P.; Halas, N. J. Acc. Chem. Res. 2007, 40, 53. (27) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (28) Sherry, L. J.; Jin, R. C.; Mirkin, C. A.; Schatz, G. C.; Van Duyne, R. P. Nano Lett. 2006, 6, 2060. (29) Sosa, I. O.; Noguez, C.; Barrera, R. G. J. Phys. Chem. B 2003, 107, 6269. (30) Melnikau, D.; Govyadinov, A. A.; Sánchez-Iglesias, A.; Grzelczak, M.; Liz-Marzán, L. M.; Rakovich, Y. P. Nano Lett. 2017, 17, 1808. (31) Yao, H.; Shiratsu, T. J. Phys. Chem. C 2017, 121, 761. (32) Artemyev, M.; Krutokhvostov, R.; Melnikau, D.; Oleinikov, V.; Sukhanova, A.; Nabiev, I. Proc. SPIE 2012, 845729. (33) Armelles, G.; Cebollada, A.; García, F.; García-Martín, A.; de Sousa, N. ACS Photonics 2016, 3, 2427. (34) Maccaferri, N.; Bergamini, L.; Pancaldi, M.; Schmidt, M. K.; Kataja, M.; Dijken, S. v.; Zabala, N.; Aizpurua, J.; Vavassori, P. Nano Lett. 2016, 16, 2533.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 23

(35) Maccaferri, N.; González-Díaz, J. B.; Bonetti, S.; Berger, A.; Kataja, M.; van Dijken, S.; Nogués, J.; Bonanni, V.; Pirzadeh, Z.; Dmitriev, A.; Åkerman, J.; Vavassori, P. Opt. Express 2013, 21, 9875. (36) Lodewijks, K.; Maccaferri, N.; Pakizeh, T.; Dumas, R. K.; Zubritskaya, I.; Åkerman, J.; Vavassori, P.; Dmitriev, A. Nano Lett. 2014, 14, 7207. (37) Alison M. Funston, C. N., Tim J. Davis, and Paul Mulvaney. Nano Lett. 2009, 9, 1651. (38) Han, B.; Zhu, Z. N.; Li, Z. T.; Zhang, W.; Tang, Z. Y. J. Am. Chem. Soc. 2014, 136, 16104. (39) Lee, A.; Ahmed, A.; dos Santos, D. P.; Coombs, N.; Park, J. I.; Gordon, R.; Brolo, A. G.; Kumacheva, E. J. Phys. Chem. C 2012, 116, 5538. (40) Lee, A.; Andrade, G. F. S.; Ahmed, A.; Souza, M. L.; Coombs, N.; Tumarkin, E.; Liu, K.; Gordon, R.; Brolo, A. G.; Kumacheva, E. J. Am. Chem. Soc. 2011, 133, 7563. (41) Zhu, Z.; Liu, W.; Li, Z.; Han, B.; Zhou, Y.; Gao, Y.; Tang, Z. ACS Nano 2012, 6, 2326. (42) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (43) Link, S.; Mohamed, M. B.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3073. (44) Chen, H.; Shao, L.; Li, Q.; Wang, J. Chem. Soc. Rev. 2013, 42, 2679. (45) Muskens, O. L.; Bachelier, G.; Fatti, N. D.; Vallée, F.; Brioude, A.; Jiang, X.; Pileni, M.-P. J. Phys. Chem. C 2008, 112, 8917. (46) Zuloaga, J.; Prodan, E.; Nordlander, P. ACS Nano 2010, 4, 5269. (47) Mack, J.; Stillman, M. J.; Kobayashi, N. Coordin. Chem. Rev. 2007, 251, 429. (48) Tserkezis, C.; Papanikolaou, N.; Almpanis, E.; Stefanou, N. Phys. Rev. B 2009, 80, 125124. (49) Jain, P. K.; Eustis, S.; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 18243.


Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Table of Contents (TOC)


Geometry-Modulated Magnetoplasmonic Optical Activity of Au

Recommend Documents