Mildred S. Dresselhaus, Institute Professor, Electrical Engineering and Computer Science (abbreviated EECS below) and Physics.
Research Staff:
Paola Corio, Postdoctoral Fellow, Physics Gene Dresselhaus, Staff Member, Francis Bitter Magnet Laboratory (FBML) Dmitry Gekhtman, Postdoctoral Fellow, Physics Ado Jorio Vasconselos, Postdoctoral Fellow, Physics Alessandra Marucci, Postdoctoral Fellow, Physics Daniel E. Oates, Visiting Scientist, Physics Herbert J. Zeiger, Visiting Scientist, Physics
Graduate Students:
Marcie R. Black, EECS Sandra D. M. Brown, Physics S. B. Cronin, Physics Y.M. Lin, EECS O. Rabin, Chemistry H. Xin, Physics Y. Habib, Physics T. Koga, Division of Applied Sciences, Harvard X, Sun, Physics Z. Zhang, Physics
Collaborators:
G. Bauer, Johannes Kepler University, Linz, Austria Gang Chen, UCLA H. M. Cheng, Institute of Metal Research, Academia Sinica, Shenyang P. C. Eklund, Pennslyvania State University M. Endo, Shinshu University, Nagano, Japan T. Enoki, Tokyo Institute of Technology, Japan T. C. Harman, MIT Lincoln Laboratory J. Heremans, Delphi Corporation, Warren, MI E. P. Ippen, EECS and Physics, MIT J. P. Issi, Catholique University, Louvain-la-Neuve, Belgium R. A. Jishi, California State University, Los Angeles K. Kneipp, Technical University, Berlin A. Marucci, CNRS, Pasis, France D. E. Oates, MIT Lincoln Laboratory R. H. Ono, NIST, Boulder, CO. M. A. Pimenta, Universidade Federal de Minas Gerais, Belo Horizonte, Brazil A. M. Rao, University of Kentucky R. Saito, Electrocommunications University, Tokyo, Japan R. E. Smalley, Rice University L. R. Vale, NIST, Boulder, CO. K. L. Wang, UCLA J. Y. Ying, Chemical Engineering, MIT
Degrees Granted:
Zhibo B. Zhang, ``Fabrication, characterization and transport properties of bismuth nanowire systems." PhD thesis, Massachusetts Institute of Technology, Department of Physics, February 1999. Y. M. Habib, ``Measurements and modeling of the microwave impedance in high-Tc grain boundary Josephson junctions: Josephson fluxon generation and vortex dynamics'', Ph.D. Department of Physics, February, 1999. Xiangzhong Sun, ``The effect of quantum confinement on the thermoelectric figure of merit'', Ph.D. Department of Physics, June, 1999. Sandra D. M. Brown, ``Resonance Raman Spectroscopy of Single-walled Carbon Nanotubes'', Ph.D. Department of Physics, June, 2000. Takaaki Koga, ``Concept and applications of carrier pocket engineering to design useful thermoelectric materials using superlattice structures'', Ph.D. Division of Engineering and Applied Sciences, Harvard University, June, 2000.Research Report:
Low Dimensional Thermoelectricity Studies Personnel: T. Koga, X. Sun, S. B. Cronin, O. Rabin, Y. M. Lin, M. R. Black, G. Chen, K. L. Wang, G. Dresselhaus, and M.S. Dresselhaus Sponsorship: US Navy, ONR, MURI A systematic procedure for designing low-dimensional superlattice structures to enhance the three-dimensional thermoelectric figure of merit has been developed. The concept, called carrier pocket engineering, was first developed by our group for the case of GaAs/AlAs superlattices and this concept has now been applied to Si/Ge superlattices where it has been possible to carry the work much further. The use of low-dimensional structures, as realized in the form of two-dimensional quantum wells and one-dimensional quantum wires, has been shown to provide a promising strategy for designing materials with an enhanced thermoelectric figure of merit ZT, arising both from quantum confinement effects and from increased boundary scattering. The original predictions of enhanced ZTs for isolated quantum wells and/or wires (denoted by Z2DT and Z1DT, respectively) used the simplest possible model based on the constant relaxation time approximation. Although these predictions were experimentally verified using the systems of (111) oriented PbTe/PbEuTe multiple-quantum-wells (MQWs) and Si/SiGe superlattices based on very thick barriers to isolate the neighboring quantum wells electrically, the design (and the experimental proof-of-principle study) of superlattices that provide enhanced values of Z3DT, where Z3DT denotes the figure of merit for the whole (3D) superlattice, has not yet been carried out. First a theoretical model was developed for selecting the superlattice geometry in order to optimize Z3DT. The superlattice structures were then built and thermoelectric transport measurements were carried out. An experimental proof-of-principle study on the use of carrier pocket engineering to enhance the thermoelectric figure of merit Z3DT at 300 K has been carried out using (001) oriented Si/Ge superlattices. The highest value of the experimentally obtained Z3DT at 300 K for (001) oriented Si/Ge superlattices, using a conservative estimate for the thermal conductivity, is 0.1, which is a factor of seven enhancement relative to the estimated value of Z3DT for bulk Si (Z3DT=0.014). The experimentally obtained values for the Seebeck coefficient agree very well with the theoretical predictions that are made, both as a function of temperature and carrier concentration. This work validates efforts to enhance Z3DT in thermoelectric materials through low-dimensional approaches. Observation of Individual Josephson Vortices generated by an external static magnetic field Personnel: H. Xin, Y. M. Habib, D. E. Oates, L. R. Vale, R. H. Ono, G. Dresselhaus and M. S. Dresselhaus Sponsorship: AFOSR
Individual Josephson vortices generated by an external dc magnetic field have been observed using engineered YBCO bi-crystal grain-boundary junctions in small dc magnetic fields (0 -- 10 Oe). The Josephson vortex dynamics is probed using a small rf signal of 4.4 GHz in a microwave-resonator setup. Each Josephson vortex entering the grain-boundary junction is manifested by a sharp peak in the microwave resistance, which is measured as the dc magnetic field is varied. Measurements are made of both the microwave resistance and reactance, and upon increasing and decreasing the dc magnetic field to examine the hysteretic behavior. The system is modeled as a long Josephson junction described by the sine-Gordon equation with the appropriate boundary conditions. Quantitative agreement is obtained between the experimental data and the model calculations for the dynamics of a long junction in the presence of dc and rf magnetic fields. The comparison between theory and experiment allows us to identify the series of sharp peaks in the microwave loss with individual Josephson vortices penetrating into the grain-boundary junctions under study.
We have also investigated the effect of the patterning process (to, for example, fabricate a stripline resonator) on the nonlinearity of the microwave surface resistance of YBCO thin films. By fabricating a sapphire dielectric resonator and a stripline resonator, the microwave surface resistance of YBCO thin films was measured before and after the patterning process as a function of temperature and of the peak magnetic field in the film. The microwave loss was also modeled by assuming a second-order dependence of the microwave surface resistance on current density. Experimental and calculated results show that the patterning of YBCO thin films has no observable effect on the microwave surface resistance. Thermal noise contributions to the Josephson vortex dynamics and the hysteretic behavior of the Josephson vortices have been studied in the context of the patterning process. The possible relation between the Josephson vortex dynamics and the patterning process has also been studied.
Resonant Raman study of Polyparaphenylene (PPP)-based carbons A. Marucci, S. D. M. Brown, M. A. Pimenta, M. J. Matthews, M. S. Dresselhaus, K. Nishimura, and M. EndoA resonant Raman study of polyparaphenylene (PPP) prepared by the Kovacic and the Yamamoto methods and heat treated at temperatures THT between 650 C and 750 C has been performed using different laser excitation energies Elaser between 1.92 eV and 3.05 eV. For samples heat treated in this range of THT, the Raman spectra change with Elaser and this behavior is ascribed to the coexistence of two forms of the PPP polymer (benzenoid and quinoid) as well as a disordered carbon material. For THT above some transition temperature, carbonization of the original polymer occurs and a difference in resonant Raman behavior is seen. This transition temperature is lower for the Yamamoto-PPP samples than for the Kovacic-PPP samples. In addition, a detailed study of the second-order Raman spectrum of the polymer polyparaphenylene (PPP) prepared according to the Kovacic method has been carried out. Several Raman bands in the region between 2400 and 3400 cm-1 have been detected and assigned to the overtones and combination bands of the two conformations of the PPP polymer (benzenoid and quinoid) that coexist in our samples. Due to the carbonization process, these bands broaden and decrease in intensity with increasing heat treatment temperature, as is also observed for the corresponding first-order Raman features. The complete absence of the high frequency Raman bands in the second order spectrum of PPP with heat treatment temperatures in excess of 750 C indicates the complete transformation of the polymer into a disordered carbon material.
Resonant Raman study of carbon nanotubes Personnel: S. D. M. Brown, A. Marucci, A. Jorio, P. Corio, K. Kneipp, M. A. Pimenta, A. M. Rao, M. Endo, G. Dresselhaus, and M. S. Dresselhaus Sponsorship: NSFThe main focus of our present research on carbon nanotubes involves a detailed study of the resonant Raman effect of the tangential G-band vibrational modes in the frequency range 1460--1620 cm-1 which provides unique information about the phonon modes, and, through the electron-phonon coupling, about the van Hove singularities in the 1D electronic density of states. By carefully selecting the laser excitation energy so that at one laser energy only semiconducting nanotubes are in resonance with the incident or scattered photons, or at another laser energy only metallic nanotubes are in resonance, it has been possible to carry out a detailed analysis of the Raman lineshapes associated with metallic and semiconducting nanotubes. It was found that the tangential G-band of the metallic nanotubes is described by a Breit-Wigner-Fano (B-W-F) lineshape, and we have studied this Raman lineshape as a function of tube diameter and laser excitation energy. Studies of the anti-Stokes spectra have been very fruitful in defining the laser excitation energy range where metallic nanotubes are resonant for the incident and scattered photons. By comparing the Stokes and anti-Stokes spectra, the nearest neighbor overlap energy or transfer integral for carbon nanotubes has been sensitively measured. The large differences between the Stokes and anti-Stokes spectra represent new findings for both Raman spectroscopy and carbon nanotubes research, and these differences in the spectra are attributed to different nanotubes being resonant with each of these processes for the same incident laser excitation energy. Polarized Raman studies have been carried out for multiwall and single wall carbon nanotubes, thereby identifying the symmetries of the various features contained within the tangential G-band for semiconducting nanotubes. Surface enhanced Raman scattering (SERS) spectra studies have been taken to increase our signal sensitivity, thereby allowing us to investigate the spectra from very small numbers of nanotubes, perhaps even a single nanotube bundle, based on measurements of linewidths close to the natural linewidth. By studying the relative intensities of the various symmetry components of the Raman G-band as identified in the polarization studies, and by studying differences in the relative intensities of these spectral features between the SERS and normal resonant Raman spectra, we have been able to identify the basic mechanism responsible for the B-W-F lineshape with a coupling of the discrete A(Ag) phonon mode that has displacements in the circumferential direction with the surface plasmon continuum. This coupling, not present in graphite by symmetry, is turned on by the curvature of the carbon nanotubes, and the experimental results show the proper dependence 1/dt2 of the mode frequency on nanotube diameter dt.
Research has also progressed on a number of topics relevant to broader issues for carbon nanotubes, such as the nearest-neighbor carbon-carbon overlap energy and trigonal warping effects in the electronic structure, and hydrogen storage in single wall nanotubes. Several large review articles have been written during the year.
Continued progress has been made in understanding the structure/property relations in other exotic forms of carbon, much of this research being done with collaborators worldwide.
Electronic Transport Properties of Single Crystal Bismuth Nanowire Arrays Personnel: Z. Zhang, X. Sun, G. Dresselhaus, M.S. Dresselhaus, J.Y. Ying, G. Chen, J. Heremans, C.M. Thrush, D.T. Morelli Y.M. Lin, S. Cronin, O. Rabin, M. R. Black. Sponsorship: US Navy, ONR MURI, NSFBismuth nanowires are of particular interest from a scientific standpoint because they undergo a semimetal-semiconductor transition as the wire diameter decreases below a critical diameter dc. The semimetal-semiconductor transition arises from quantum confinement effects in bismuth which is a semimetal in bulk form. Quantum confinement cause the lowest L point conduction subband edge to move up in energy and the highest T point valence subband edge to move down, thus yielding a one-dimensional (1D) semiconductor below some critical wire diameter dc. At very small nanowire diameters, well within the semiconducting regime, the Bi nanowires offer promise for thermoelectric applications. For both scientific and practical reasons, the transport properties of Bi nanowires are of great interest.
To explain various temperature-dependent resistivity measurements R(T) on bismuth (Bi) nanowires as a function of wire diameter, a semi-classical transport model has been developed, which explicitly considers anisotropic and non-parabolic carriers in cylindrical quantum Bi nanowires. The first numerical solutions for cylindrical quantum wires have been obtained. The temperature-dependent resistivity, carrier density, Seebeck coefficient, power factor, and thermoelectric figure of merit Z1DT have been calculated for circular Bi nanowires with various wire diameters and crystalline orientations. The results show the trigonal axis is the most favorable wire orientation for thermoelectric applications, and Z1DT > 1 is predicted for n-type trigonal wires with diameters dw < 10 nm. The effect of the T-point holes on Z1DT has also been investigated. It is found that Z1DT can be significantly enhanced, especially for p-type Bi nanowires, if the T-point holes are removed or suppressed. In these calculations the relative importance of various scattering processes have been considered on the basis of simple assumptions. The T dependence of the resistance is found to be strongly affected by the semimetal-semiconductor transition.
Experimental results have been obtained for R(T) for undoped Bi nanowires of 40 nm diameter for several electron concentrations using Te doping. The calculation described above provides a good model for interpreting these data.
The first resistance measurements on a single Bi nanowire have been made. The measurement techniques are being improved to make temperature-dependence measurements, and measurements as a function of wire diameter. Antimony doping techniques are being developed for the future fabrication of p-type Bi nanowires. Optical property measurement and analysis techniques are being developed to characterize the electronic properties of the nanowires in more detail.