Probing the electromagnetic field of a 15-nanometre hotspot by single molecule imaging.
When light illuminates a rough metallic surface, hotspots can appear, where the light is concentrated on the nanometre scale, producing an intense electromagnetic field. This phenomenon, called the surface enhancement effect, has a broad range of potential applications, such as the detection of weak chemical signals. Hotspots are believed to be associated with localized electromagnetic modes, caused by the randomness of the surface texture. Probing the electromagnetic field of the hotspots would offer much insight towards uncovering the mechanism generating the enhancement; however, it requires a spatial resolution of 1-2 nm, which has been a long-standing challenge in optics. The resolution of an optical microscope is limited to about half the wavelength of the incident light, approximately 200-300 nm. Although current state-of-the-art techniques, including near-field scanning optical microscopy, electron energy-loss spectroscopy, cathode luminescence imaging and two-photon photoemission imaging have subwavelength resolution, they either introduce a non-negligible amount of perturbation, complicating interpretation of the data, or operate only in a vacuum. As a result, after more than 30 years since the discovery of the surface enhancement effect, how the local field is distributed remains unknown. Here we present a technique that uses Brownian motion of single molecules to probe the local field. It enables two-dimensional imaging of the fluorescence enhancement profile of single hotspots on the surfaces of aluminium thin films and silver nanoparticle clusters, with accuracy down to 1.2 nm. Strong fluorescence enhancements, up to 54 and 136 times respectively, are observed in those two systems. This strong enhancement indicates that the local field, which decays exponentially from the peak of a hotspot, dominates the fluorescence enhancement profile.
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
Hu
H
Cang
Anna
A
Labno
Changgui
C
Lu
Xiaobo
X
Yin
Ming
M
Liu
Christopher
C
Gladden
Yongmin
Y
Liu
Xiang
X
Zhang
D000535:Aluminum / Q000737:chemistry; D004574:Electromagnetic Fields; D005453:Fluorescence; D005456:Fluorescent Dyes / Q000032:analysis / Q000737:chemistry; D006358:Hot Temperature; D008163:Luminescent Measurements / Q000379:methods*; D053768:Metal Nanoparticles / Q000737:chemistry*; D008856:Microscopy, Fluorescence / Q000379:methods*; D057054:Molecular Imaging / Q000379:methods*; D009038:Motion; D011791:Quartz; D012834:Silver / Q000737:chemistry; D013499:Surface Properties
D005456:Fluorescent Dyes; D011791:Quartz; D012834:Silver; D000535:Aluminum
Nano Lett. 2007 Sep;7(9):2843-6
17718531
Nat Methods. 2010 May;7(5):377-81
20364147
Science. 1997 Feb 21;275(5303):1102-6
9027306
Phys Rev Lett. 2001 Oct 15;87(16):167401
11690242
Nano Lett. 2008 Apr;8(4):1159-62
18321073
Angew Chem Int Ed Engl. 2009;48(49):9285-9
19890928
Phys Rev Lett. 2006 Mar 24;96(11):113002
16605818
Proc Natl Acad Sci U S A. 2006 Dec 12;103(50):18911-6
17142314
Science. 2006 Sep 15;313(5793):1642-5
16902090
Nat Methods. 2006 Oct;3(10):793-5
16896339
Nano Lett. 2009 Mar;9(3):995-1001
19199757
Science. 1992 Jul 10;257(5067):189-95
17794749
Phys Rev Lett. 2006 Nov 17;97(20):206103
17155697