- Post Doctoral
MIT Unit Affiliation:
- Electrical Engineering & Computer Science
Post Doc Sponsor / Advisor:
Date PhD Completed:
Top 3 Areas of Expertise:
Physicist researcher with extensive experience in solid state physics, microfabrication and characterization, Raman spectroscopy, CVD and laboratory research. Excellent researcher with proven ability to resolve problems independently and actively contribute to the research projects goals. Possess a proven publication track record and a good standard of written and oral communication. Strong ability to interact and collaborate in a team environment on multi-disciplinary projects in a constructive, creative and professional manner. Passionate about learning scientific skills and experienced in managing multiple projects simultaneously.
Expected End Date of Post Doctoral Position:
- Electrochemical delamination process of transfer of CVD grown graphene to transparent and flexible substrates for opto-electronic devices.
- Growth of CVD graphene on Ni single crystal substrate with different crystallographic orientations.
- PECVD growth and characterization of graphene on tungsten/silicon substrates.
- Electrochemical exfoliation of graphene and Bi2Te3 for aerogel fabrication and application.
In the last decade, many theoretical and experimental achievements have been made
in the physics of graphene. In particular, Raman spectroscopy has been playing an important
role in unraveling the properties of graphene systems. In this thesis we use the
Raman spectroscopy to study some effects of the electron-phonon coupling in monolayer
and bilayer graphene and to probe the electronic and vibrational structure of bilayer
graphene. Phonon self-energy corrections have mostly been studied theoretically and experimentally
for phonon modes with zone-center (q = 0) wavevectors. Here, we combine
Raman spectroscopy and gate voltage to study phonons of monolayer graphene for the
features originated from a double-resonant Raman (DRR) process with q ̸= 0 wavevectors.
We observe phonon renormalization effects in which there is a softening of the frequency
and a broadening of the decay width with increasing the gate voltage, that is opposite
from what is observed for the zone-center q = 0 case. We show that this renormalization
is a signature for the phonons with q ≈ 2k wavevector that come from both intravalley
and intervalley DRR processes. Within this framework, we resolve the identification of
the phonon modes contributing to the G⋆ Raman feature, at ∼ 2450 cm−1, and also for
five second order Raman combination modes in the frequency range of 1700 − 2300 cm−1
of monolayer graphene. By combining the DRR theory with the anomalous phonon renormalization
effect, we show a new technique for using Raman spectroscopy to identify the
proper phonon mode assignment for each combination mode.
We also study the behavior of the optical phonon modes in bilayer graphene devices
by applying top gate voltage, using Raman scattering. We observe the splitting of the
Raman G band as we tune the Fermi level of the sample, which is explained in terms of
mixing of the Raman (Eg) and infrared (Eu) phonon modes, due to different doping in the
two layers. We show that the comparison between the experiment and theoretical model
not only gives information about the total charge concentration in the bilayer graphene
device, but also allows to separately quantify the amount of unintentional charge coming
from the top and the bottom of the system, and therefore to characterize the intrinsic
charges of bilayer graphene with its surrounding environment.
In the second part of this thesis, the dispersion of electrons and phonons near
the K point of bilayer graphene was investigated in a resonant Raman study of the
G′ band using different laser excitation energies in the near-infrared and visible range.
The electronic structure was analyzed within the tight-binding approximation, and the
Slonczewski-Weiss-McClure (SWM) parameters were obtained from the analysis of the
dispersive behavior of the G′ band considering both the inner and the outer DRR processes.
We show that the SWM parameters obtained considering the inner process are
in better agreement with those obtained from other experimental techniques, strongly
suggesting that the inner process is the main responsible for the G′ feature in graphene.
Additionally, the dependence of the intensity of the four peaks that compose the G′
band of bilayer graphene with laser excitation energy and laser power is explored and explained
in terms of the electron-phonon coupling and the relaxation of the photon-excited
electron. We show that the carrier relaxation occurs predominantly by emitting a lowenergy
acoustic phonon and the different combinations of relaxation processes determine
the relative intensities of the four peaks that give rise to the G′ band. Some peaks show
an increase of their intensity at the expense of others, thereby making the intensity of
the peaks both different from each other and dependent on laser excitation energy and
on power level. This effect gives important information about the electron and phonon
dynamics and needs to be taken into account for certain applications of bilayer graphene
in the field of nanotechnology.
Top 5 Awards and honors (name of award, date received):
5 Recent Papers:
Mafra, D.L.; Araujo, P.T. (2014) "Intra- and Interlayer Electron-Phonon Interactions in 12/12C and 12/13C BiLayer Graphene." Applied Sciences, 4 (2), 207-239.
Araujo, P.T.; Frank, O.; Mafra, D. L.; Fang, W.; Kong, J.; Dresselhaus, M. S. ; Kalbac, M. (2013), "Mass-related inversion symmetry breaking and phonon self-energy renormalization in isotopically labeled AB-stacked bilayer graphene." Nature Scientific Reports, 3, 2061.
Mafra, D. L.; Kong, J.; Sato, K.; Saito, R.; Dresselhaus, M. S. ; Araujo, P.T., (2012), "Using the G' Raman cross-section to understand the phonon dynamics in bilayer graphene systems." Nano Letters, 12, 2883-2887.
Araujo, P. T.* ; Mafra, D. L.* ; Sato, K. ; Saito, R. ; Kong, J. ; Dresselhaus, M. S. (2012), "Phonon self-energy corrections to non-zero wavevector phonon modes in single-layer graphene." Physical Review Letters, 109, 046801. * Those authors contributted equally for this work.
Araujo, P.T. ; Mafra, D. L. ; Sato, K.; Saito, R.; Kong, J.; Dresselhaus, M. S. (2012). "Unraveling the interlayer-related phonon self-energy renormalization in bilayer graphene." Nature Scientific Reports, 2, 1017.