Visible light emission and metal-semiconductor transition in single walled carbon nanotube composites T. Pradeep Department of Chemistry and

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1 Visible light emission and metal-semiconductor transition in single walled carbon nanotube composites T. Pradeep Department of Chemistry and Sophisticated Analytical Instrument Facility Indian Institute of Technology Madras Chennai Recent trends in molecular materials research, Kovalam January 20-22, 2008

2 Acknowledgement C. Subramaniam CSIR Fellowship Collaborators (Takuji Ogawa, IMS) Nanoscience and Nanotechnology Initiative of the DST

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4 Confocal Raman Microscope MALDI TOF MS Transmission Electron Microscope QTrap MS Nanoscience and Nanotechnology Initiative of the DST Ultramicrotome

5 Au Monolayers 0 S S Structure, phase transitions Reactivity, applications 2 nm Nanotube-nanoparticle Anisotropic structures

6 Prof. K. Kimuara

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8 a b c d Shibu, Nishida et al 2008

9 In this talk.. Visible emission from single walled carbon nanotubes Background Experiment Control experiments What next

10 (n,m) indices Metallic Semiconducting

11 Electrical transport properties Semiconducting : (n-m) = 3l. Eg = ev Metallic : (n-m) = 3l. E g = ev Semiconducting Metallic ChemPhysChem, 2005, 5, 577.

12 Emission spectrum (red) of individual fullerene nanotubes suspended in SDS micelles in D 2 O excited by 8 ns, 532-nm laser pulses, overlaid with the absorption spectrum (blue) of the sample in this region of first van Hove band gap transitions. Science, 2002, 297, 593.

13 Vibrational Properties (A) Radial Breathing Mode (RBM) Diameter dependent RBM ( cm 1 ) d ( nm) t 12.5 (B) Tangential G band In-plane vibrations (C) D-band Defect centered (D) G` band Occurs at 2ω D Intensity (arb. units) SWNTs powder Science, 1997, 275, 187. & Raman Shift (cm -1 )

14 Vibrational properties of single walled carbon nanotubes (SWNTs) Experimental methods Confocal Raman spectral analysis and imaging Scanning near field optical microscopy (SNOM) Point-contact current image Atomic force microscopy (PCI-AFM). Other supporting experiments

15 Purification of SWNT SWNT Sigma Aldrich (pre-purified) CNI (HiPCo) Sonication and centrifugation 1. Annealed 2. Acid treated 3. Neutralised 4. Washed and dried Centrifugate Purified SWNT

16 Preparation of composite Diethyl ether SWNT in DMF Diethyl ether Aqueous Nanoparticles M NPs/NRs M NPs Au/Ag nanoparticles, Citrate synthesized M NRs Au nanorods Au NPs

17 Types of systems investigated SWNT mswnt Au nanoparticles Ag nanoparticles Au nanorods NP-SWNT NR-mSWNT 100 nm 2 nm www1.eere.energy.gov 20 nm 5 nm

18 Transmission Electron Microscopy 0.1 μm 0.1 μm TEM images of Au-SWNTs composite acquired at 100 kev.

19 10 nm TEM images of (A) AuNRs-SWNTs composite acquired at 300 kev.

20 52k (A) 40k (B) 50k Intensity (arb. units) 48k 46k 44k Intensity (arb. units) 38k 36k 42k Binding energy (ev) 34k Binding Energy (ev) XPS spectra of Au-SWNT composite in the (A) Ni 2p and (B) Fe 2p regions. ICP - MS

21 (A) (B) Excitation wavelength (nm) Emission wavelength (nm) Emission wavelength (nm) Fluorescence contour plots of (A) supernatant solution and (B) blank water at ph 7.12.

22 Instrumentation Confocal Raman Concept of confocality Raman Instrument

23 Key instrument specifications Argon Ion laser : nm Back scattering geometry Super notch filter

24 Scanning Near-field Optical Microscopy Resolution is limited by wavelength of light used. Near-filed microscopy was first proposed by Synge in Resolutions below the diffraction limit can be obtained when the tip-sample distance is smaller than the aperture diameter. In such a case, the aperture diameter controls the resolution and not the wavelength of light used Phil. Mag., 1928, 6, 356.

25 Intensity (arb. units) k 20k 15k 10k Wavelength (nm) k Wavelength (nm) (ii) (a) (b) (c) 669 2k 1k k G` band (d) Raman Shift (cm -1 ) (e) Raman Shift (cm -1 ) (i) Raman Spectra of (a) Ag-SWNTs composite, (b) Au-SWNTs composite, (c) AuNR-SWNTs composite, (d) pristine SWNTs, (e) Pristine SWNTs treated with trisodium citrate and (f) Au nanorods.

26 Raman Spectral imaging Intensity (arb. units) 25k 20k 15k 10k 5k Raman Shift (cm -1 ) 7 μm

27 Varying excitation sources a b Wavelength (nm) Excitation : 532 nm a b Excitation : 633 nm c c Raman Shift (cm -1 ) Raman Shift (cm -1 ) Raman Spectra acquired with (A) 532 nm Nd-YAG and (B) 633 nm He-Ne as excitation sources. Traces (a), (b) and (c) correspond to Ag-SWNTs composite, Au-SWNTs composite and AuNR-SWNTs composite, respectively.

28 5 μm (A) SNOM images of Au-SWNT composite along with the (C) topography. (B) and (D) are their three dimensional representations.

29 (A) (B) Transmission SNOM images of pristine SWNT based on (A) topography and (B) light intensity.

30 Supporting experiments Intensity (arb.units) (A) C CTAB = 10-6 M C CTAB = 10-5 M C CTAB = 10-4 M C CTAB = 10-3 M C CTAB = 10-2 M C CTAB = 10-1 M (B) Raman Shift (cm -1 ) 100 nm (A) Raman spectra of Ag-SWNT composite measured as a function of CTAB concentration. (B) TEM image of Au-CTAB-SWNT at C CTAB = 10-4 M

31 90 (A) Au 4f 7/2 105 (B) Ag 3d 5/2 Intensity (arb. units) Au 4f 5/2 Intensity (arb. units) Ag 3d 3/ Binding energy (ev) Binding Energy (ev) XPS spectra of (A) Au-SWNT and (B) Ag-SWNT composites in the Au 4f and Ag 3d regions, respectively

32 Intensity (arb. units) 25k 20k 15k 10k 5k (A) Intensity (arb. units) 20k 10k (B) C SWNT = 1.7 mg/ml C SWNT /2 C SWNT /3 C SWNT /4 C SWNT /5 C SWNT / Raman Shift (cm -1 ) Raman Shift (cm -1 ) Raman spectra of Ag-SWNT composite, measured as a function of (A) concentration of Ag nanoparticles, (B) SWNT concentration.

33 (n,m) indexing d t RBM 3 C 1 C constants 2, where C1 and C2 are d a c c t n 2 nm m 2 (n,m) RBM d t (nm) (cm -1,Theoretical) (10,10) (18,0) (13,7) (17,0) (11,9) (12,8) m E 11 s s E / 23 E32

34 What we know so far Visible fluorescence from SWNTs is demonstrated. Raman spectral mapping is done to ascertain the origin of fluorescence. SNOM of SWNT structures is done using this fluorescence.

35 Origin of visible fluorescence Near-infrared fluorescence in isolated SWNT is known. This is not observed in bundles and metallic SWNTs. Metallic SWNTs offer non-radiative decay channels. So what happens to the metallic SWNTs present in the composite?

36 Separation protocol SWNT + isopropyl amine + THF Ultrasonicate, 1h SWNT dispersion 40,000 g, 16 0 C, 12h Centrifugate Residue J. Am. Chem. Soc., 2005, 127, Alkyl amines stabilize metallic SWNT

37 Estimating metallic content Point Contact Current Imaging AFM (PCI-AFM) Mica/Insulating substrate Thermo evaporation of gold Quartz microbalance To pump Gold Resistive wires

38 (A) Measurement geometry (B) (C) Nanotube Mica/insulating substrate (A) Photograph of the scanning head, (B) schematic of the PCI-AFM measurement and (C) representative AFM image of pristine SWNTs

39 (A) Current (na) (B) (C) Bias Voltage (V) Electrode is 200 nm away from the image-edge 100 nm Conductivity (na/v) Bias Voltage (V) (A) PCI-AFM images of pure mswnt with (B) I-V curves and (C) plot of conductance versus bias voltage.

40 (A) 40 (B) Current (na) Bias Voltage (V) Current (na) nm 200 nm Bias Voltage (V) (C) 400 (D) Current (na) Current (na) nm Bias Voltage (V) nm Bias Voltage (V)

41 (E) 100 nm (F) 1 Current (na) Bias Voltage (V) Current (na) Bias Voltage (V) 1 G1 G b 4 1a 2a 3a 4a 2b 5a 3b 6a 4b 5b G4 6b G2 (G) 200 nm

42 10 (B) 5 (A) Electrode is 420 nm away from the edge nm 2 Conductance (na/v) Current (na) Bias Voltage (V) (C) Bias Voltage (V) (A) PCI-AFM image of Au-mSWNT with (B) the corresponding I-V curves and (C) Plot of conductance versus bias voltage. 1 Au nanoparticle Carbon nanotubes

43 80 60 Bias Voltage (V) Pure mswnt Au-mSWNT Conductance (na/v) Conductance (na/v) Bias Voltage (V) Comparison of conductance versus bias voltage for pure mswnt and AumSWNT composite

44 G-band line shapes of metallic and semiconducting SWNT Changes observed in G-band for metallic (left) and pristine (right) SWNT Science, 2003, 301, 345.

45 Confocal Raman investigations 700 Intensity (arb. units) A s /A m = 65.0 A s /A m = 0.14 Extraction efficiency A s /(A s + A m )* 100 = 88% Raman Shift (cm -1 ) Raman spectra in RBM and G-band regions of pristine SWNT (black) and extracted mswnt (red).

46 Pure mswnt FWHM = 48 cm -1 Au- mswnt FWHM = 22 cm Raman Shift (cm -1 ) Raman Shift (cm -1 ) Comparison of G-band of pure mswnt and Au-mSWNT

47 Conclusions Metallicity of SWNT is destroyed by interaction with nanoparticles. PCI-AFM and confocal Raman confirm this M-S transition. mswnt fluoresce when their metallicity is destroyed. M-S transition has far reaching implication in nanoelectronics and design of nanodevices. C. Subramaniam et al. Phys. Rev. Lett. (2007) C. Subramaniam and T. Pradeep Patent applications 2005, 2006, 2007

48 Application of fluorescence in gas sensing and separation

49 Fuel Cells : Methanol fuel cells, metal- hydride fuel cells, Gas storage materials Carbon nanotubes 1600 m 2 /g 1 5% of publications relate to gas storage 1 1 Acc. Chem. Res., 2007, DOI : /ar700013c

50 Specific adsorption sites Interstitial site Endohedral Site Groove site External site Loadings from 0.08 wt% to 12 wt % have been reported

51 Experimental setup To rotary pump To Hg manometer To gas source To rotary pump To Raman spectrometer Fibre optic pin-hole detection Super-notch filter Open/close valve Sample compartment Sample

52 Intensity (arb. units) 6k 5k 4k 3k 2k Raman Shift (cm -1 ) 10 torr 20 torr 30 torr 40 torr 60 torr 100 torr 140 torr 175 torr 200 torr 220 torr 250 torr 275 torr 320 torr 350 torr Au-SWNTs exposed to He gas at various partial pressures.

53 4.0k 0 torr Intensity 3.5k 3.0k 2.5k 2.0k 550 torr 1.5k Raman Shift (cm -1 ) Au-SWNTs exposed to H 2 gas at various partial pressures.

54 Fluorescence intensity (a.u.) % Groove sites Endohedral & interstitial sites 32% External sites, 9% Pressure of hydrogen (torr)

55 5.0k 4.5k 0 torr 500 torr Intenisty (arb. units) 4.0k 3.5k 3.0k 2.5k 2.0k 1.5k Raman Shift (cm -1 ) Au-SWNTs exposed to N 2 gas at various partial pressures.

56 Gas separation possibility Intensity (arb. units) 3.0k 2.5k 2.0k 1.5k 1.0k In vacuum 100 torr H torr H torr N torr removed In vacuum Raman Shift (cm -1 )

57 Intensity (arb. units) In vacuum 100 torr N torr N torr H torr gas removed In vacuum Raman Shift (cm -1 )

58 Intensity (arb. units) 4.5k 4.0k 3.5k 3.0k 2.5k 2.0k 1.5k 1.0k Raman Shift (cm -1 ) Vacuum 10 torr 20 torr 30 torr 40 torr 50 torr 75 torr 100 torr 125 torr 150 torr 175 torr 200 torr 225 torr 250 torr 275 torr 300 torr 325 torr 350 torr 375 torr 400 torr 425 torr 450 torr 475 torr Methane on Au-SWNTs

59 Intensity (arb. units) 3.5k 3.0k 2.5k 2.0k 1.5k 1.0k Vacuum 10 torr 20 torr 30 torr 40 torr 50 torr 60 torr 75 torr 100 torr Raman Shift (cm -1 ) n-hexane on Au-SWNTs

60 Intensity (arb. units) Raman Shift (cm -1 ) Cyclohexane on Au-SWNTs Vacuum 10 torr 20 torr 40 torr 50 torr 60 torr Vacuum

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62 5 1 1 II I

63 5 I Current(nA) Au-mSWNT with N Bias Voltage (V)

64 80 I Conductance (na/v) Au-mSWNT with N Bias Voltage (V)

65 20 I 10 Current (na) 0-10 Au-mSWNT with H Bias Voltage (V)

66 50 40 I Au-mSWNT with H 2 Conductance (na/v) Bias Voltage (V)

67 50 Conductance (na/v) Au-mSWNT + Nitrogen Au-mSWNT + Hydrogen Bias voltage (V) Plot of conductance versus bias voltage constructed at various point of Figure 1A, under an atmosphere of nitrogen (red traces) and hydrogen (black traces).

68 (B) Intensity (a.u.) (c) (d) (b) (a) (b) (d) (c) (a) 1800 Raman Shift (cm -1 ) Raman spectra of (a) purified mswnts, (b) Au-mSWNT composite, (c) Au-mSWNT upon exposure to 500 torr H 2 and (d) Au-mSWNT composite after pumping out H 2 exposed in (c). Spectra (a) to (d) are recorded at the same point on the composite sample.

69 Conclusions and future directions Gas adsorption inside SWNT was studied using visible fluorescence from Au-SWNTs composite Behavior was similar in case of Ag-SWNT and AuNR-SWNT composites. In-situ gas storage detection Observation of different adsorption sites Gas separation Understanding gas storage inside SWNTs Probing electronic structure variation upon gas adsorption Possibilities of isomer separation C. Subramaniam and T. Pradeep Patent application 2007

70 Thank you all IIT Madras