Current Research Projects

The overall goal of our research program is to understand the fundamental electronic and chemical properties of materials of scientific and technological importance.

 

  Four classes of materials are presently being studied:

Correlated Materials Organic Superconductors

Wide Band Gap Semiconductors Organic Semiconductors

  Numerous experimental techniques are used:

Primary Techniques: Angle Resolved Photoelectron Spectroscopy (ARPES) Soft X-Ray Emission (SXE) Resonant Inelastic X-Ray Scattering (RIXS) X-Ray Photoemission Spectroscopy (XPS) X-Ray Absorption Spectroscopy (XAS)

Secondary Techniques: Low Energy Electron Diffraction (LEED) Auger Electron Spectroscopy (AES) Mass Spectroscopy Atomic Force Microscopy (AFM) Scanning Electron Microscopy (SEM)

  Experiments are performed around the world:

• The National Synchrotron Light Source, Brookhaven National Laboratory, New York;
• The Advanced Light Source, at Lawrence Berkeley National Laboratory, California;
• The MAXLAB synchrotron, in Lund, Sweden;
• The Novel Materials Laboratory at Boston University.

  Our research is funded by:

• The U.S. Department of Energy
• The National Science Foundation
• The U.S. Army Research Office
• The Petroleum Research Fund

1. Correlated and Low Dimensional Solids

Correlated and low dimensional solids present important challenges to modern solid state physics. While our knowledge of the origin the physical properties of many simple solids is both comprehensive and sophisticated, this is not the case in low dimensional and correlated solids. From quasi-low dimensional conductors, through magnetoresistive oxides, to high temperature superconductors, there exists a plethora of physical phenomena displayed by these materials that remain poorly understood.  Our program explores the basic electronic structure of a variety of low dimensional and correlated solids. The primary tools used are synchrotron radiation-excited high resolution soft x-ray emission spectroscopy, resonant inelastic x-ray scattering, and high resolution photoemission spectroscopy.   Among the specific goals of our program are to use these techniques to:

• study the coupling between collective excitations and quasi-particles;
• probe spin-charge separation and non-Fermi liquid phenomena in reduced dimensional solids;
• measure the element resolved electronic structure of correlated materials;
• measure Fermi surface structures in quasi-low dimensional conductors;
• measure changes in electronic structure in low-dimensional and correlated solids during metal to non-metal transitions;
• enhance scientific infrastructure by further developing resonant inelastic soft x-ray scattering and soft x-ray emission spectroscopy as probes of electronic structure in complex materials

2. Organic Superconductors

Organic superconductors have been the subject of intense study due to the challenge they pose to our understanding of the physical properties of complex solids and due to technological interest in developing advanced carbon-based electronic devices. Charge transfer salts are organic superconductors that have received much attention. Aside from superconductivity, they exhibit a host of fascinating phenomena including quasi-low dimensional charge transport, metal-insulator transitions, charge density waves, spin density waves, and Peierls transitions. While the origin of these phenomena lies in the electronic structure of the solids, there is a remarkable lack of detailed spectroscopic measurements of this structure. This is primarily due to severe synchrotron radiation beam damage and difficulties in preparing stoichiometric surfaces. Photoemission spectroscopy is a ubiquitous probe of electronic structure in solids, but photoemission studies of organic superconductors have met with limited success, and many important properties, such as Fermi surface structures, the density of states close to the Fermi level (E_F), and the nature of defects, remain poorly understood. Our program aims to use a powerful combination of high resolution synchrotron radiation-based soft x-ray spectroscopies to measure the electronic structure of organic superconductors. We solve the beam damage and surface problems associated with earlier studies of single crystals by growing thin films of these materials in-situ, and continuously translating the films in front of the synchrotron radiation beam as spectra are recorded. We are working towards two significant breakthroughs: definitive spectroscopic measurements of electronic structure in organic superconductors, and the development of methods to synthesize thin films of organic superconductors using organic molecular beam deposition techniques.  The primary spectroscopic probes to be used in this program will be high resolution soft x-ray emission spectroscopy, resonant inelastic x-ray scattering, high resolution core and valence band photoemission spectroscopy, and soft x-ray absorption. Among the specific goals of the program are:

• to develop organic molecular beam deposition methods for the in-situ synthesis of ordered thin films of organic superconductors on various substrates;
• to measure the density of states, band structure, and orbital bonding of thin film organic superconductors; and
• to study the growth, structure, and stability of overlayers on organic superconductors with the aim of understanding the formation of contact layers.

3. Wide Band Gap Semiconductors

Wide band gap nitride semiconductors are the focus of intense scientific scrutiny due to their numerous potential applications in optoelectronic and high temperature devices. However, our understanding of the basic physics of these materials lags far behind our ability to make simple electronic devices from them.  If wide band gap nitride semiconductors are to achieve their full technological potential then we must have as deep an understanding of their fundamental properties as exists for Si and III-V electronic materials.   Our program aims at understanding the bulk and interface electronic structure of nitride semiconductors, the electronic structure of clean and adsorbate-covered nitride semiconductor surfaces, and the electronic structure of metal and non-metal overlayers on these nitride surfaces. Among our specific goals are:

• to study the surfaces of clean nitride semiconductor alloys in order to understand their fundamental electronic properties, including the properties of surface defects;
• to study molecular adsorption on well characterized nitride semiconductor surfaces in order to understand the chemical reactivity of the surfaces, their passivation, and the growth, structure, and stability of overlayers, including organic thin films;
• to study the bulk and buried interface electronic structure in nitride semiconductors;
• to study the electronic structure of model magnetic semiconductors such as GaMnN

4. Thin Film Organic Semiconductors

Organic semiconductors have been the subject of intense study due to technological interest in developing carbon-based electronic devices, and due to the challenge they pose to our understanding of the physical properties of complex solids. It is also clear that successful development of carbon-based electronics would have significant long term implications for the use of fossil fuels. While much progress has been made in the synthesis and doping of thin film organic semiconductors, there is a remarkable lack of comprehensive, detailed measurements of electronic structure in these materials. Our program aims at the in-situ study of the electronic structure of thin film organic semiconductors using a variety of synchrotron radiation-based spectroscopic probes. The primary tools will be high resolution soft x-ray emission spectroscopy, high resolution photoemission spectroscopy, and soft x-ray absorption. Among our specific goals are:

• to measure the density of states, band structure, and orbital bonding of thin film organic semiconductors;
• to study the growth, structure, and stability of overlayers on organic semiconductors with the aim of understanding the formation of contact layers;
• to investigate the nature and severity of photon- and electron-induced beam damage.

Comprehensive study of the electronic structure of in-situ grown films using synchrotron radiation spectroscopies has never been attempted before.