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Feng Wang, Yuanbo Zhang, Chuansan Tian , Caglar Girit , Alex Zettl , Michael Crommie , Y. Ron Shen Presenter: Sumit Mondal. Outline. Brief overview of Infrared spectroscopy Experimental details Measurement on single layer graphene Observations and discussion

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Outline

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  1. Feng Wang, Yuanbo Zhang, ChuansanTian, CaglarGirit, Alex Zettl, Michael Crommie, Y. Ron Shen Presenter:SumitMondal

  2. Outline • Brief overview of Infrared spectroscopy • Experimental details • Measurement on single layer graphene • Observations and discussion • Measurement on bilayer • Observations and discussions • Weaknesses / Comments • Conclusion • References

  3. Infrared Spectroscopy • Range of infrared radiation: roughly 13000 to 10 Divided into three regions – far, mid and near infrared • Infrared activity of a sample is often associated with the vibrational modes of the molecules • If there is a net dipole moment change in the vibration, the mode is infrared active • Infrared Reflectance spectroscopy is the study of light as a function of wavelength that has been reflected or scattered from a sample • This can give information about various absorptions taking place in the sample

  4. Infrared spectroscopy can be quite informative in case of graphene as most of interesting energy scales including band structure parameters in graphene fall in this range • Infrared Spectroscopy has been used to probe interband Optical transitions in monolayer and bilayerGraphene • Unlike conventional materials , the Optical transition in graphene can be modified dramatically using electrical gating • The special behavior is due to its 2D structure that confines the electron on one atomic layer and its low density of states near Dirac point, which causes the Fermi energy to shift significantly with changing carrier density

  5. Experimental details • Mechanically exfoliated graphene monolayer and bilayer prepared on heavily doped by peeling ( thickness 290nm) • Monolayers and bilayers identified by Optical microscope and confirmed by Raman Spectroscopy • Gold electrode (thickness ~ 30nm) attached by deposition in vacuum

  6. IR Reflection spectroscopy performed on the sample at room temperature • Tunable IR radiation generated by femto-second Ti/Sapphire amplifier system • The back reflected light was collected and detected by a mercury cadmium telluride detector • Typical lateral dimension of the sample 20 • Infrared beam was confined to a diameter <10 on the sample

  7. Measurements • Two types of measurements were performed 1) Normalized change of IR reflectivity from the sample with respect to the bare substrate 2)To study the gate dependence , sample position was fixed and a DC gate was applied A small AC modulation was attached to the gate, the modulated change in reflectivity at the ac modulation frequency was detected

  8. Optical microscope image of monolayer and bilayer

  9. Normalized change in IR Reflectivitywith reference to bare substrate

  10. The graphene monolayer spectrum is featureless because of it’s linear electronic bands The absorption coefficient of an undopedgraphene in the Infrared region should be a constant (ref. 17, 18) The normalized change in reflectance picks a slow wavelength dependence For a bilayer, the optical transitions between parallel valence bands lead to a strong van Hove Singularity Clear peak near 350meV, near the parallel band separation of 400meV found by photoemission (ref. 21, 23)

  11. Gate dependence of Optical transitionsTwo dimensional plots of (A) measured and (B) calculated versus gate voltage and probing photon energy for a Graphene monolayer(Calculation was done under tight binding approximation)

  12. Observations For a given photon energy the absolute value of has a maximum at certain gate voltage (can be seen from the vertical cut) The gate voltage for the maximum signaldecreases with increasing photon energy (red line) The band dispersion can be directly probed from the transitions Change in the charge-carrier density by the gate voltage: ; is the offset voltage caused by natural doping Corresponding shifted

  13. The band filling effect dominates at photon energy corresponding to transition that originates from the states near Fermi surface So the maximum signal is defined by From the slope of , dispersion velocity of the Dirac band is calculated to be Value is comparable with transport measurements [1,3] and Photoemission [23-25]

  14. Gate dependence of Optical transitionsTwo dimensional plots of (A) measured and (B) calculated versus gate voltage and probing photon energy for a Graphenebilayer(Calculation was done under tight binding approximation)

  15. Observations For a give photon energy the absolute value of has a clear peak in the modulation spectra (can be seen from the horizontal cut) The signal at the peak has an opposite sign as compared to the monolayer indicating a fundamentally different mechanism Transition between the parallel valence bands of hole doped bilayer give rise to van Hove singularity and the observed spectral peak The peak is at the band separation energy and becomes stronger with increasing hole concentration as shifts down allowing more and more transition Other interband transitions contribute to a relatively flat response over the spectral region

  16. Weaknesses/Comments Slight differences in the peak energy in the observed and the calculated data at spectral range from 370 to 400meV Observed peak energy ~ 350meV is close but smaller than the band separation energy 400meV determined by Photoemission [21,23] These discrepancies may have arised from strong many body effects in Graphenebilayer. For instance, excitonic effects can modify the optical spectrum appreciably, whereas photoemission probes the quassiparticle transitions at the absence of excitons I don’t understand why don’t they talk about the Transmittance also as both are needed to find out the complex optical conductivity required for the theoretical plot

  17. Conclusions The observed interband transitions are quite strong. Although one atom or two atom thick, graphene monolayer and bilayer can absorb 2% to 6% of incident IR radiation Ungated Monolayer spectrum is almost featureless. In case of gated monolayer, for a given photon energy the absolute value of has a maximum at certain gate voltage For a gated bilayer, the has a clear peak in both gate voltage and photon energy Transition between the parallel valence bands of hole doped bilayer give rise to van Hove singularity and the observed spectral peak For bilayer a small band gap is expected to appear because of asymmetry of gating effect of the upper and lower layer [21,27] B y looking at the high absorptoin ratio and gate controlled tunability one might envision one might envision novel graphene-based optoelectronic devices such as tunable IR detectors, modulators and emitters

  18. Thank you!

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