Reference sample
For TERS Imaging Report by the TERS-Team within the COST Action MP1302 Nanospectroscopy
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July 2017
Raul D. Rodriguez*1,2 Diana Davila Pineda3 Lina Dedelaite4 Ashutosh Mukherjee2 Evgeniya Sheremet1,2 Dietrich R.T. Zahn2
Contents Overview 3 Fast Facts
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Analysis
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Micro Raman characterization under two different wavelengths Micro Raman imaging 6
Strategy
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TERS imaging: Seeing is believing 7 Characteristics of an ideal reference sample
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TERS quality checklist 8 TERS Quality Checklist
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Anex 10 Au nanocrystal tips for AFM-based TERS
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Affiliations and contact: 1. 2. 3. 4.
Tomsk Polytechnic University, Tomsk, Russia Semiconductor Physics, Chemnitz University of Technology, Chemnitz, Germany IBM Zurich Research Lab, Zurich, Switzerland Department of Physical Chemistry, Vilnius University, Vilnius, Lithuania
*
[email protected]
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Reference samples for TERS imaging
Overview In collaboration with IBM Zurich we have produced dozens of essentially identical samples. The samples have different arrays of Au nanostructures uniformly functionalized with a Raman active probe. These nanostructures are intended as reference samples for TERS imaging in the first inter-laboratory study of this kind.
Figure 1: Sketch for the reference sample for TERS imaging. a) The Au nanodisks, with a constant height of 20 nm, were deposited on a Au layer of 50 nm thickness supported on a Si wafer with a 3 nm Cr adhesion layer. The samples were coated by an ultra thin layer 2 nm thick of cobalt phthalocyanine. b) There are 4 fields in which the interparticle distance changes from 50, 80, 100, and 150 nm, from left to right. In each field, there are 5x4 arrays. All rows in each field are identical, changing nano disk size along the columns. www.ters-team.com
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Reference samples for TERS imaging
The individual Au nanoparticles cannot be resolved with conventional optical micro spectroscopy. Therefore, the principle of this study is very simple: To confirm whether or not one has TERS by resolving (or not) the individual nanostructures with a size well below the diffraction limit of light.
Fast Facts
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Dozens of samples containing each different arrays of Au nano disks were fabricated at IBM Zurich using e-beam lithography. An ultra-thin film of cobalt phthalocyanine (CoPc) with a nominal thickness of 2 nm was deposited simultaneously on all samples by organic molecular beam deposition under ultrahigh vacuum conditions. A silicon substrate with native oxide layer was also used for deposition in order to obtain the non-enhanced spectrum. The samples were characterized by micro Raman spectroscopy under green and red laser excitations, 514.7 nm and 632.8 nm, respectively. Different inter-particle distance and particle size allows optimizing the maximum enhancement to be in resonance with the different laser lines used by the participants. Multiple TERS imaging sessions on different samples with different Au nano-crystalline tips in AFM feedback were performed. A red laser excitation (638 nm) and a commercial TERS system with side illumination/collection optics (100x, N.A. 0.7) were used. Sub-diffraction imaging with TERS allowed resolving the individual Au nanoparticles in the arrays. The well-defined nanoparticle arrays were verified to be stable for TERS imaging over a period of several weeks.
Analysis One of the functionalized samples was analyzed with conventional micro Raman spectroscopy. Spectra from all the structures were obtained in order to determine the different spectral regions for enhancement on a single chip sample with different nanoparticle arrays. The results are summarized in Figure 2 for the intensity of the CoPc peak at 1535 cm-1 (the same peak is used in all the intensity maps herein). Using the highest magnification lens available in our lab (100x, N.A. 0.9), in the backscattering
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Reference samples for TERS imaging configuration. As expected, the Au nano disks were not resolved by micro Raman imaging. The hyper-spectral TERS imaging with 1ms acquisition time per spectrum performed on different samples with different tips confirmed the suitability of this systems as standards for TERS imaging.
Micro Raman characterization under two different wavelengths In the charts below the intensity enhancement comparison for different fields on a single chip under two different laser excitations are shown.
Raman excitation 514.7 nm
Raman excitation 632.8 nm
Figure 2: Intensity enhancement of the 1535 cm-1 mode from CoPc for different array size and inter-particle distances. The first value of each couple number in the x axis gives the particle diameter, while the second number gives the inter-particle separation. Two typical wavelengths used in TERS were used as indicated in each graph, that allows the user to chose a resonant excitation with the sample. The rightmost bar in each graph represents the non-enhanced intensity obtained for CoPc on Si. www.ters-team.com
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Reference samples for TERS imaging
Micro Raman imaging
Figure 3: Micro Raman spectroscopy imaging results obtained under red laser excitation (632.8 nm) and 100x objective (N.A. 0.9) in the backscattering configuration. a) Raman spectra averaged over the flat Au film and over the region covered by Au nanodisks in the 100-100 nm array shown in the optical image in b). c) Cross-section profile of the intensity variation of the CoPc mode at 1535 cm-1 over the Au nano disks/flat interface shown in the Raman map in d). The Raman intensity map is from the region shown by a rectangle in b).
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Reference samples for TERS imaging
Strategy TERS imaging: Seeing is believing Since the Au particles are too small to be resolved by conventional optics, as shown in Figure 3, we can claim TERS if we show a Raman intensity map that resolves the individual nanoparticles. This simple check goes beyond the usual tip up/tip down comparisons that are highly dependent on the far-field tip scattering signal (non-TERS), contamination of the tip and other artifacts that make questionable the result. An example is shown in Figure 4 for a TERS map acquired over a region on the 50-250 nm array.
Figure 4: a) TERS map and topography b) with a pixel size of 7 nm resolving the individual nanodisks. c) spectral comparison between two points 7 nm apart from each other marked by arrows in a). d) Zoom out showing multiple particles in the array and the height image in e). www.ters-team.com
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Reference samples for TERS imaging
Characteristics of an ideal reference sample One sample to fit the instrument and configuration of the Round Robin participants Wish list: reference sample for TERS imaging Raman active Chemically and mechanically stable Heterogeneous, with well-defined nanoscale features Easily accessible (commercially available) Compatible with STM, AFM, tuning fork Compatible with transmission, side, and top illumination configurations
TERS quality checklist This checklist (an initiative from ES) contains key information that must be filled and returned back with your TERS results. (attached as a word file) Analogous to the "Solar Checklist" introduced by Nature aiming "to aid reproducibility and transparency of results", at the WG3 meeting in Warsaw we proposed to introduce a checklist for nanospectroscopy techniques. The idea is that the authors associated with the COST Action MP1302 will fill in and submit this checklist to the publishers to be passed further to the reviewers. The information could be included in the methods section of the manuscript or in supplementary information. Alternatively, filled checklist may be published as a supplementary material if the authors wish so. It was agreed that the first checklist will be drafted for TERS and refined during the Round Robin on the TERS reference sample.
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Reference samples for TERS imaging
TERS Quality Checklist Checklist item
Response
1. Have tip-up and tip-down spectra been provided?
N.A.
2. Has subdiffraction resolution been demonstrated with TERS imaging? What is the spatial resolution achieved?
3. Illumination parameters. Please include the following: a. Laser wavelength b. Laser power c. Laser spot size (preferably experimentally measured) d. Illumination and collection geometry 4. Feedback mechanism. Please include: a. SPM method used b. Tip material and preparation method c. Typical tip-sample separation distance and imaging mode
Yes, 7 nm
a. b. c. d.
a. AFM b. Au nanocrystals c. Tapping, 10 nm amplitude
5. Enhancement factor (EF). Please describe how the EF has been calculated?
6. Optical properties of the tip and the sample. If known, please include: a. LSPR data for the tip b. Substrate material and its effect on the LSPR of the tip c. Information on the optical resonance of the sample d. Was it in gap-mode? 7. Spectral analysis. a. Please state the spectral resolution and spectra acquisition time b. Have you observed any changes in the TERS spectra as compared to micro-Raman spectra? c. Have you observed any changes in the spectra or topography after the TERS measurements? Please describe them if so.
638 nm 1.3 mW N.A. 0.7 Side at 60 degrees
N.A.
a. b. c. d.
PL emission ca. 680 nm Au nanodisks Variable (See Figure 2) Yes
a. ca. 6 cm-1, 1ms b. Occasional variation in the Raman peak intensity ratios c. Only once when dozens of images were acquired until the image quality deteriorated due to tip changes (after 4 days of continuous scanning)
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Reference samples for TERS imaging
Anex Au nanocrystal tips for AFM-based TERS All the results obtained in these analyses were performed with our latest technology developed for the production of TERS-active tips. We obtain a high degree of reproducibility with yield close to 100% in TERS imaging. This is possible by combining the best of well-established micro fabrication methods for Si cantilevers with the strong plasmonic enhancement of Au single nanocrystals. Here below is an example, original Si probes by Sergey Lemeshko from NT-MDT and Vladilena Daly from SpectrumInstruments.
Figure 5: Spectroscopic analysis of a tip before a) and b), and after c) and d) growth of Au nanocrystals (NC). a) and c) are the intensity maps of the Si peak around 520 cm-1, and b) and d) are the intensity maps of the plasmonic luminescence emission. e) Spectra of the tip before and after Au NC growth shown the PL emission and the Raman peaks from Si marked by *. The insets are optical microscopy images of the tip before and after, the Au NC are visible as a small shiny speckle at the tip apex.
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